Drought Creates a Wild Year in Weed Management

By July, if farmers were thinking about spraying anything on their fields, water was probably at the top of the list.

Even though weed-resistance management took a backseat to the drought throughout most of the Corn Belt, the issue isn't going away. From a resistance management standpoint, Southern Illinois University weed specialist Bryan Young says the heat and lack of rain did some good and bad things this year.

Increased awareness meant a lot of farmers went back to soil residual herbicides applied pre-emergence. The lack of rain meant many of these herbicides didn't perform very well. Young says many were hoping for 90% control, realistically, they got more like 10% control.

Bryan Young says there are certain situations where herbicide carryover could be a concern.

However, not all was lost. Many of those who applied a pre-emergence introduced another mode of action into the herbicide program. This is a definite win for resistance management, Young notes.

Post-emergence herbicides were also less effective this year because of the heat and drought stress. On the other hand, weed regrowth was reduced significantly for the same reasons, Young adds.

The biggest downer deals with weed populations that were injured by herbicides, but not fully killed. Young says drought years are bad for exposing weed populations to this type of low-level resistance scenario.

For those looking to plant wheat this fall, herbicide carryover could be a concern. Young says he wouldn't get too worried about it. But, high-pH fields that saw late-season, high-dose applications of atrazine could be a problem. He recommends a soil test or applying a bio-acid to raise the pH.

As expected, glyphosate-resistant populations of Palmer amaranth continue to spread across Illinois. Young notes Collinsville is the northernmost location with commercial levels of problematic Palmer amaranth.

To protect against herbicide-resistant Palmer amaranth, Young recommends a program that involves multiple modes of action and a liberal use of pre-emergence herbicides. The trick is starting with a clean field. To do so, farmers need to get in the habit of spraying much smaller weeds. Young says three inches is the new target height. The days of a one-size-fits all glyphosate approach are over.